A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852, which is incorporated herein by reference in its entirety) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors, and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than this surface of the projection system). One way which has been proposed to arrange for this is disclosed in WO 99/49504, which is incorporated herein by reference in its entirety. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the projection system, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2, the liquid is supplied along the direction of movement of the substrate relative to the projection system, although this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the projection system are possible. One example is illustrated in FIG. 3, in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the projection system.
Another solution which has been proposed is to provide the liquid supply system with a seal member which extends along at least a part of a boundary of the space between the projection system and the substrate table. Such a solution is illustrated in FIG. 4. The seal member 12′ is substantially stationary relative to the projection system PL in the XY plane, although there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member 12′ and the surface of the substrate W. Preferably the seal is a contactless seal such as a gas seal. Such as system with a gas seal is disclosed in European Patent Application No. 03252955.4, which is incorporated herein by reference in its entirety.
In European Patent Application No. 03257072.3, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without immersion liquid, and exposure is carried out with a stage at a second position, where immersion liquid is present. Alternatively, the apparatus has only one stage.
Although providing improved resolution, the introduction of an immersion liquid has been found to cause errors in the image generated on the substrate, including alignment errors between one layer and the next (i.e. overlay errors), defocus, and aberrations. It is, therefore, desirable to provide a system that reduces lithography errors arising from the immersion liquid.